In recent years, a new type of gasoline combustion has been demonstrated. In particular, a pre-mixed air-fuel mixture is compressed in a combustion chamber such that the mixture combusts without using a spark plug to initiate the combustion. This type of combustion has been developed to improve fuel economy and emissions of gasoline engines. HCCI combustion may result in higher thermal efficiency as compared to common spark ignition (SI) combustion. This is because HCCI combustion is initiated at a plurality of sites in the combustion chamber. Combustion starts by auto-ignition and occurs simultaneously at a number of sites in the cylinder rather than at a single source. This sequence is different from the SI combustion which begins at the spark plug and then propagates to the combustion chamber periphery as the flame front progresses. Further, HCCI combustion may also increase the cylinder mixture temperature when the pre-mixture is lean or when the pre-mixture is diluted by EGR. The cylinder mixture can be heated such that auto-ignition is more likely to occur when the cylinder is compressed. If the cylinder mixture is not pre-heated before compression at lower engine speeds and load, auto-ignition is less likely to occur. HCCI combustion also reduces NOx formation in the cylinder because the peak cylinder pressure is reduced. However, as mentioned above, when the engine is operating at lower speeds and lower loads, the pre-mixed air-fuel temperature may not increase to auto-ignition temperature even when the piston reaches the top-dead-center position.
One example of a method of operating a gasoline engine using HCCI combustion is described by U.S. Pat. No. 6,425,367. The method describes auto-ignition of a mixture that is facilitated by providing a negative overlap period wherein both of an intake and an exhaust valve are closed. This increases the temperature inside of the combustion chamber because a higher quantity of exhaust gases is retained in the combustion chamber. Auto-ignition is further promoted by producing an active air-fuel mixture that has high ignition performance. This is achieved by injecting a part of the fuel directly into the combustion chamber in the negative overlap period. This process causes the injected fuel to evaporate immediately into the higher temperature exhaust gases. As a result, the fuel is broken down into radical molecules having broken molecular chains, or the fuel can be oxidized into an aldehyde, both of which may promote auto-ignition as the piston approaches top-dead-center.
However, the inventors herein have recognized that there is room for further improvement of this example. Specifically, at engine operating conditions where combustion chamber temperature is low, thermal efficiency or engine emissions can degrade since auto-ignition may not occur at the appropriate time, even if injecting a part of fuel is implemented in the negative overlap period.
Another example of a method of operating a gasoline engine using HCCI combustion is described by U.S. Pat. No. 7,234,438. This patent describes spark-assisted HCCI combustion. Specifically, the method comprises bringing the temperature of the combustion chamber close to auto-ignition temperature by adjusting engine operating conditions. In one embodiment, a small cloud of stratified air-fuel mixture is formed near the spark plug. The fuel cloud is ignited by a spark from the spark plug. This action causes cylinder pressure to rise, thereby producing auto-ignition at other sites in the cylinder. This method also describes dividing engine operation into three different combustion modes that are determined with respect to engine speed and load. HCCI mode is operational at lower engine speeds and loads and appears to be surrounded by a spark assisted HCCI mode region. Further, the spark ignition (SI) combustion mode appears to be reserved for areas of higher engine speeds and loads as well as engine speeds and loads that are lower than those reserved for spark assisted HCCI mode region. According to the method described in U.S. Pat. No. 7,234,438, spark assisted HCCI combustion is used at both of lower and higher engine speed and load conditions than the engine speed and load conditions where HCCI combustion with no spark-assist is implemented.
However, the inventors herein have recognized that the amount of NOx produced by the engine at lower speeds and loads can be undesirable if spark assisted HCCI is implemented as described in this patent.
One embodiment of the present description includes method to operate an internal combustion engine having a combustion chamber with a piston and a spark plug, the method comprising: during a first mode, bringing the temperature of the combustion chamber to auto-ignition temperature by adjusting engine operating conditions and producing auto-ignition in said combustion chamber without requiring spark from said spark plug; and during a second mode, bringing the temperature of the combustion chamber close to auto-ignition temperature by adjusting engine operating conditions, forming a small cloud of stratified air-fuel mixture near said spark plug, igniting said fuel cloud by a spark form said spark plug, and then causing cylinder pressure to rise, thereby producing auto-ignition at other sites in said combustion chamber wherein said first mode is implemented in a first operating range and said second mode is implemented only in a second operating range where engine speed and load are lower than said first operating range.
This method overcomes at least some of the disadvantages of the prior art.
Thermal efficiency or engine emissions can be improved by using spark assisted HCCI combustion in low engine speed and load conditions. Further, by implementing spark assisted HCCI only at lower engine speed and load conditions than HCCI combustion, NOx production by igniting stratified air-fuel mixture can be decreased under relative high engine speed and load conditions while keeping HCCI combustion stable.
A second embodiment of the present description includes a method to operate an internal combustion engine having a combustion chamber with a piston and a spark plug, the method comprising: during a first mode, bringing the temperature of the combustion chamber to auto-ignition temperature by adjusting engine operating conditions and producing auto-ignition in said combustion chamber without requiring spark from said spark plug; during a second mode, bringing the temperature of the combustion chamber close to auto-ignition temperature by adjusting engine operating conditions, forming a small cloud of stratified air-fuel mixture near said spark plug, igniting said fuel cloud by a spark from said spark plug, and then causing cylinder pressure to rise, thereby producing auto-ignition at other sites in said combustion chamber; and during a third mode, producing substantially homogenous air-fuel mixture having substantially stoichiometric air fuel ratio is produced in said combustion chamber and igniting said substantially homogenous air-fuel mixture by a spark form said spark plug, wherein said first mode is implemented in a first operating range, said second mode is implemented only in a second operating range where engine speed and load are lower than said first operating range and said third mode is implemented in a third operating range where engine speed and load are lower than said second operating range, and wherein, for a predetermined time period in transition between said second mode and said third mode, only stratified air-fuel mixture is produced in said combustion chamber by providing fuel into said combustion chamber directly in compression stroke and said stratified air-fuel mixture is ignited by a spark form said spark plug.
This method also overcomes at least some of the disadvantages of the prior art and has further advantage.
Thermal efficiency and engine emissions can be improved by using spark assisted HCCI combustion in low engine speed and load conditions. Further, by implementing spark assisted HCCI only at lower engine speed and load conditions than HCCI combustion, NOx production by igniting stratified air-fuel mixture can be decreased under relative high engine speed and load conditions while keeping HCCI combustion stable. Further, the engine combustion stability during mode transition between spark assisted HCCI combustion mode and SI mode where engine speed and load are too low to bring the temperature of the combustion chamber to auto ignition temperature even if spark assist is used can be improved by producing only stratified air-fuel mixture in the combustion chamber by providing fuel into the combustion chamber directly in compression stroke and igniting the stratified air-fuel mixture by a spark form said spark plug.